Traumatic brain injury (TBI) is considered to be a major health problem in the United States. Approximately every 16 seconds in the United States, a person suffers a TBI. The CDC estimates that 1.5 to 2 million individuals experience TBI annually, resulting in 1.4 million emergency department visits, 235,000 hospitalizations and 52,000 deaths. (Coronado V G, XU L, Basavaraju S V, McGuire L C, Wald M M, Faul M D, et al. Surveillance for traumatic brain injury-related deaths—United States, 1997-2007. MMWR Surveill Summ. 2011. 60(5):1-32). TBI is particularly devastating among young adults and children, making it the leading cause of death and disability in the United States. Almost half a million emergency department visits for TBI are made annually by children aged 0 to 14 years (Langlois J A, Rutland-Brown W, Walk M M. J. Head Trauma Rehabil. 2006. 21(5):357-8; Satz P, Zauch K, McCleary C, Light R, Asarnow R, Becker D. Psychol Bull. 1997: 122(2):107-31). In addition, according to a recent CDC report, Americans 65 and older experience a high rate of TBI due to the incidence of falls (Thomas K E, Stevens J A, Sarmiento K, Wald M M. 2005. J. Safety Res. 2008; 39(3):269-72). TBI also ranks at the top of common traumatic events involving the central nervous system (CNS), outpacing traumatic spinal cord injury by a factor of 10. The socioeconomic consequence of TBI can be quite significant; considering the injury costs associated with expenditures for acute treatment, long-term rehabilitation, and loss of productivity (Shi J, Xiang H, Wheeler K, Smith G A, Stallones L, Groner J, et al. Brain inj. 2009. 23(7):602-11). Despite TBI's major public health issues, resources allocated for TBI research (from basic science to clinical research) at the national level have not been met with the same level of urgency.
The Blood Brain Barrier (BBB)
The BBB is defined as the boundary that regulates access of blood components (influx of nutrients and efflux of waste) and immune cells into the brain (Abbott N J, Ronnback L, Hansson E. Nat Rev Neurosci. 2006. 7(1):41-53). Brain endothelial cells are connected by tight intercellular junctions (TJ) that provide the physical barrier characteristic of the BBB. Tight junction formation appears early in brain development greatly restricting the paracellular movement of solutes (water-soluble and polar compounds) and small ions into the brain (Anstrom J A, Thore C R, Moody D M, Brown W R. Histochem Cell Biol. 2007. 127(2):205-13). The BBB is conceptually part of a larger structure termed the neurovascular unit, which is composed of endothelial cells, glial cells, pericytes and neurons. Under physiological conditions, the role of the BBB is to protect and maintain the delicate neuronal environment. Disruption of the BBB leaves the CNS vulnerable to neuronal damage. In fact, BBB dysfunction is observed in virtually all aspects of neurodegeneration including: neurotrauma (stroke, head injury, hemorrhage), neuropathogenesis (viral, bacterial and parasitic), neurological disease (Alzheimer's disease, Multiple Sclerosis, Parkinson's disease), epilepsy and brain tumors (Persidsky Y, Ramirez S H, Haorah J, Kanmogne G D. J Neuroimmune Pharmacol. 2006. 1(3):223-36; Zlokovic B V. Neuron. 2008. 57(2):178-201; Oby E, Janigro D. Epilepsia. 2006. 47 (11): 1761-74).
Brain endothelial tight junction complexes at the molecular level are composed of the following proteins: occludin, claudin (claudin-3, 5, 12), Zonula Occludens protein (ZO-1, 2, 3), and the junctional adhesion molecules (JAM-A, B, C) (Abbott N J, Ronnback L, Hansson E. Nat Rev Neurosci. 2006. 7(1):41-53). The assembly of the tight junction is such that the intracellular ZO proteins form the major anchoring site for transmembranous occludin, claudin and JAM proteins to bind. In addition, via adaptor proteins the tight junction complex is bridged to the cytoskeleton of the cell. The tight junction complex is central to endothelial-endothelial adhesion and BBB integrity.
Injury to the BBB by Injury to the Brain
Injury to the brain such as that sustained in TBI damages cerebral blood vessels which circumvents the barrier mechanisms of the BBB and thus allows blood contents to leak into the brain (Hicks R R, Baldwin S A, Scheff S W. Mol Chem Neuropathol. 1997. 32(1-3):1-16; Signoretti S, Vagnozzi R, Tavazzi B, Lazzarino G. Neurosurg Focus. 2010. 29(5):E1). Once leakage occurs, secondary events are triggered that contribute to further injury. At the end of the acute phase of TBI, leakage at the direct site of impact seals due to the natural clotting process. However, after a period of time the blood vessels in and around the lesion become more permeable than those in undamaged areas. This hyper-permeability leads to 1) a loss of control over the neurochemical environment around neurons and 2) entry of potentially toxic molecules into the CNS that exacerbates secondary damage (Hall E D, Bryant Y D, Cho W, Sullivan P G. J Neurotrauma. 2008. 25(3):235-47; Bazarian J J, Wong T, Harris M, Leahey N, Mookerjee S, Dombovy M. Brain Inj. 1999. 13(3):173-89).
Blast Mediated Brain Injury
Blast induced neurotrauma (BINT) is a type of TBI that can occur when an individual is exposed to the blast wave of a detonated explosive. (Cernak, I and Noble-Haeusslein, L. J. J Cereb Blood Flow Metab 2010. 30:255-266). Because of the nature of modern warfare, military personnel engaged in conflict areas are at particular risk of experiencing a BINT event. (Cernak, I. Front Neurol 2010. 1:151). These injuries occur as a result of an impact by a shock wave with properties of increased pressure, heat and density. The damage is exacerbated by the prolonged negative pressure phase following immediately behind the shock wave front. Blast overexposure (BOP) generate acute damage to gas-filled organs (i.e. lungs) and solid organs. In the brain BINT causes distinct pathological changes as a direct result of the passing blast wave that accelerates and/or rotates the brain. (Pun, P B et al. Front Neurol 2011. 2:19). In addition, BOP causes enhanced vascular loading from the transfer of kinetic energy to the body's fluid phase (mainly the blood). Vascular loading causes oscillating fluidic waves that induce changes to the cerebrovasculature which lead to increase in BBB permeability, edema formation, vasospasms and altered blood flow. The neurological symptoms of BMBI range widely depending on the degree of injury and can manifest in months to years after exposure to the blast. (Svetlov S I, Prima V, Kirk D R, Gutierrez H, Curley K C, Hayes R L and Wang K K. J. Trauma 2010. 69:795-804).
Epilepsy and Seizures
Epilepsy is diagnosed when an individual experiences repeated convulsions over a given period of time. (Oby, E and Janigro D. Epilepsia 2006. 47:1761-1774). Not always involving convulsions, seizures are episodes of abnormal electrical activity in the brain which can manifest as changes in attention or behavior. Common causes of epilepsy include conginetal brain defects, infections, stroke, TBI, metabolic disorders and brain tumors. (van Vliet E A, et al. Brain 2007. 130: 521-534). A strong correlation exists between BBB disruption and seizures. Analysis of small molecular tracers that enter the brain when the BBB is disrupted has been shown in both humans and in animal studies of epilepsy. (van Vliet E A, et al. Brain 2007. 130: 521-534). Furthermore, abnormal electroencephalogram (EEG) patterns can be observed when there is hyper-permeability of the BBB. In fact serological studies in patients with epilepsy have shown the presence of neuronal and glial proteins (that normally are not present in the blood) as a consequence of BBB deregulation in epilepsy. While several methods can be used to determine directly the proper function of the BBB in animal models, a fitting (i.e., intravital microscopy, histology or analytical) technique for evaluating BBB integrity in humans does not exist.
Stroke
Stroke is the 4th leading cause of death in the United States, affecting more than 750,000 individuals per year. (Ovbiagele B and Nguyen-Huynh M N. Neurotherapeutics 2011. 8:319-329). Stroke denotes a sudden disruption or stoppage of blood flow in the brain which subsequently deprives brain tissue of oxygen and nutrients. The interruption in blood flow can occur as a result of a blood clot blockage (ischemic stroke) or rupture (hemorrhagic stroke) of a cerebral blood vessel. (Lo E H, Dalkara T and Moskowitz M A. Nat Rev Neurosci 2003. 4:399-415). After the onset of stroke, edema formation develops and induces a rise in intracranial pressure which can lead to compression, herniation and damage of brain tissue. Increase in cerebrovascular permeability due to BBB disruption is a critical factor in the development of edema. (Jiang Q et al. J. Cereb Blood Flow Metab 2005. 25:583-592). Often the edema that forms worsens during the phase of reperfusion. Inflammatory mediators and cellular proteins from injured cells activate the endothelium and augments BBB permeability contributing not only to edema formation but to the disruption in neuronal homeostasis. (Cipolla M J, Huang Q and Sweet J G. Stroke 2011. 42:3252-3257).
Inflammatory and Infectious Diseases of the Brain
Many infectious diseases affecting the brain cause changes to the brain vasculature that often lead to a breach of the BBB. Examples of these types of diseases include viral infections caused by HIV-1, Rabies, cerebral malaria, and Japanese encephalitis virus. (Persidsky, Y et al. J Immunol 1997. 158:3499-3510; Fabis M J, Phares T W, Kean R B, Koprowski H and Hooper D C. Proc Natl Acad Sci USA 2008. 105:15511-15516; Tripathi A K, Sha W, Shulaev V, Stins M F and Sullivan D J, Jr. Blood 2009. 114: 4243-4252; Liu T H, Liang L C, Wang C C, Liu H C and Chen, W J. J. Neurovirol 2008. 14: 514-521). Also bacterial infections caused by Escherichia coli K1, group B streptococcus, Listeria monocytogenes, Citrobacter freundii and Streptococcus pneumonia strains have been shown to affect the BBB. (Huang S H, Stins M F and Kim K S. Microbes Infect 2000. 2:1237-1244). Under inflammatory conditions, the normal function of the BBB is compromised due to overproduction of pro-inflammatory molecules by inflammatory cells. Whether induced by trauma (i.e TBI), cerebrovascular accident (stroke), a pathogen or neurological disorder (i.e multiple sclerosis, Alzheimer's disease) the breach of the BBB is significantly driven by the up-regulation of inflammatory pathways in activated cells of the neurovascular unit and by the recruitment of immune cells. (Persidsky Y and Ramirez S H. In The Neurology of AIDS (Gendelman H E, Grant I, Everall I P, Lipton S A and Swindells S, eds) pp. 220-230. Oxford University Press, New York). BBB disruption is markedly enhanced by the recruitment of immune cells to the brain endothelium in a process that involves immune adhesion and transendothelial migration. Therefore BBB injury in neuroinflammation is believed to result from the disruption of junction complexes between brain microvascular endothelial cells that facilitates the diffusion of blood products and entry of leukocytes into the brain parenchyma.
Method for the Detection of Injury to the Brain
Until now most efforts on TBI serum biomarkers have centered on detecting neuronal and glial proteins that have leaked from the brain into the blood (Menascu S, Brezner A, Tschechmer S M, Rumeny P G. Pediatr Neurosurg. 2010. 46(2):82-8). Such candidate neuronal and glial proteins include: S100β, Tau, NSE (neuron-specific enolase), SBDPs (spectrin breakdown products), ApoE (apolipoprotein E) and GFAP (glial fibrillary acidic protein). Although there have been promising results, these candidate biomarkers have not proven entirely useful for the diagnosis of brain injury. All of these candidate biomarkers have been poor predictors of long-term outcome after TBI, some lacking specificity or sensitivity.
There remains a need for an objective, blood based diagnostic test and for an effective pharmacological (or biological) treatment for TBI, and other brain injuries.